linux-kernel - Re: [PATCH] xen-pciback: Consider MSI-X enabled only when MASKALL bit is cleared

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Date:   Thu, 17 Nov 2022 12:54:51 +0000
From:   David Vrabel <dvrabel@...tab.net>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, linux-kernel@...r.kernel.org
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Jan Beulich <jbeulich@...e.com>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] xen-pciback: Consider MSI-X enabled only when MASKALL bit
 is cleared

On 17/11/2022 11:41, Marek Marczykowski-Górecki wrote:
> Linux enables MSI-X before disabling INTx, but keeps MSI-X masked until
> the table is filled. Then it disables INTx just before clearing MASKALL
> bit. Currently this approach is rejected by xen-pciback.
> Allow setting PCI_MSIX_FLAGS_ENABLE while INTx is still enabled as long
> as PCI_MSIX_FLAGS_MASKALL is set too.

The use of MSI-X interrupts is conditional on only the MSI-X Enable bit. 
Setting MSI-X Enable effectively overrides the Interrupt Disable bit in 
the Command register.

PCIe 6.0.1 section 7.7.2.2. "MSI-X Enable ... is prohibited from using 
INTx interrupts (if implemented)." And there is similar wording for MSI 
Enable.

I think you need to shuffle the checks for MSI/MSI-X in 
xen_pcibk_get_interrupt_type() so they are _before_ the check of the 
Interrupt Disable bit in the Command register.

David